Widely tunable semiconductor lasers are important elements for next generation optical communications systems and possibly for other applications such as testing, biomedical, inspection, etc.
Current methods embedded in widely tunable semiconductor lasers are generally divided into: (1) externally tuned lasers (external cavity laser—ECL) and lasers utilizing a monolithical solution based on a distributed Bragg reflector laser (DBR)—see Table 1. In the ECL, the laser cavity is comprised of a semiconductor chip and external spectrally sensitive elements (mainly gratings) that serve as an out of chip wavelength selective mirror. Tuning of the laser is performed by rotating or modifying the external gratings (by applying an external field such as heat, stress, etc.). This type of tunable laser is problematic due to the packaging and environmental reliability of the hybrid device, and is used mainly as a laboratory device and not in optical communications systems. In lasers utilizing a monolithical solution (based on a DBR), all the laser parts are realized on a single chip. Here, two generic solutions exist:
(a) The two laser mirrors are made of sampled (SGDBR) or super structure gratings (SSGDBR) each to generate a spectral sequence of high transmission peaks (spectral comb). This is schematically illustrated in FIG. 1. The two combs of the two mirrors can be aligned by current injection such that a spectral peak of one mirror overlaps the spectral line of the other (Vernier tuning) [G. Sarlet, G. Monthier, R. Baets, “Wavelength and mode stabilization of widely tunable SG-DBR and SSG-DBRlasers”, IEEE Photonics Technology Letters, Vol. 11 no. 11 1999 pp 1351].
(b) Grating-Assisted Codirectional Coupler with Sampled Reflector (GCSR). One laser mirror is comprised of sampled or super structure gratings and the laser active region is coupled to this mirror via a narrow bandpass filter (realized as a long period grating assisted coupler). This is schematically illustrated in FIG. 2. The tuning is performed by current injection to the sampled, superstructure gratings and the bandpass filter is tuned (also by current injection) to overlap one of the spectral reflectivity peaks of the mirror.